IIRC, part of the GHz problem is that very long pipelines like that of the Pentium 4 tend to show increasing benefits at higher clocks. If you can keep the pipeline full then the system reaps the benefits. Sort of like a drag racer - goes very fast in a straight line but terrible on corners.

But with longer pipelines comes larger penalties when the pipeline needs to be flushed, so the P4 eventually hit a wall and Intel returned to the late Pentium 3 Tualatin core, refining it into the Pentium M which later evolved into the first Core CPUs.

only just last year did someone goose a PC cpu to 9.13ghz

https://www.tomshardware.com/pc-components/cpus/core-i9-1490...